The blast furnace is a gas-liquid-solid counter-current chemical reactor, the main purpose of which is the production of cast iron, subsequently converted to steel by reducing its carbon content.
The blast furnace is fed with solid materials, mainly with sinter, pellets, iron ore and coke, in its upper part. The liquids consisting of cast iron and slag are discharged at the hearth in its lower part.
The conversion of the iron-containing charge (sinter, pellets and iron ore) to cast iron is conventionally carried out by reduction of the iron oxides by a reducing gas (in particular containing CO, H2 and N2), which is formed by combustion of coke at the tuyeres located in the bottom part of the blast furnace where air preheated to a temperature between 1000° C. and 1300° C., called hot blast, is injected.
In order to increase the productivity and reduce the costs, auxiliary fuels are also injected at the tuyeres, such as coal in pulverized form, fuel oil, natural gas or other fuels, combined with oxygen which enriches the hot blast.
The gases recovered in the upper part of the blast furnace, called throat gases, mainly consist of CO, CO2, H2 and N2 in respective proportions of around 22%, 22%, 3% and 53%. These gases are generally used in other parts of the plant as fuel. Blast furnaces are therefore significant producers of CO2.
In view of the considerable increase in the concentration of CO2 in the atmosphere since the beginning of the last century, it is essential to reduce emissions of CO2 where it is produced in a large quantity, and therefore in particular at blast furnaces.
For this purpose, during the last 50 years, the consumption of reducing agents in the blast furnace has been reduced by half so that, at present, in blast furnaces of conventional configuration, the consumption of carbon has reached a low limit linked to the laws of thermodynamics.
One known way of additionally reducing CO2 emissions is to reintroduce throat gases that are purified of CO2 and that are rich in CO into the blast furnace. The use of CO-rich gas as a reducing agent thus makes it possible to reduce the coke consumption and therefore the CO2 emissions.
In one preferred configuration, CO is reintroduced at two levels, on the one hand level with the tuyeres at a temperature of around 1200° C., more largely between 1000° C. and 1300° C. and, on the other hand, level with the waist, in the vicinity of the waist-stack angle of the blast furnace, at a temperature of around 900° C., more largely between 1000° C. and 1300° C. This known system is described with reference to FIG. 1.
The blast furnace 1 is fed with coke, with sinter, with pellets, and with iron ore 2 via the line 3 at point 4. The cast iron and the slag 5 are recovered at point 6 level with the hearth via the line 7. Oxygen and coal and/or other auxiliary reducing agents 8 are introduced at point 9 level with the tuyeres via the line 10.
The throat gases are recovered at point 11 of the upper part of the blast furnace by means of the line 12. One portion 13 of these throat gases is exported via the pipe 14 into another device of the site. The other portion of the throat gases is recycled into the blast furnace by means of the pipe 15.
This portion of the throat gases intended to be recycled is purified of most of its CO2 by means of a CO2 purifier. This purifier 16 may, for example, consist of a physicochemical absorption process using a solution of amines, or a pressure swing adsorption (PSA) process or a vacuum pressure swing adsorption (VPSA) process, these processes possibly or possibly not being combined with a supplementary cryogenic step intended to produce pure CO2 17 ready to be stored in subsoils (this then refers to geological storage) or to be used in specific applications such as the food industry or the enhanced recovery of hydrocarbons from deposits in the final stage of extraction.
The CO-rich gas 18 is then heated in heat exchangers 24, commonly referred to as ‘cowpers’, then introduced into the blast furnace 1 at a temperature between 700° C. and 1000° C. at point 20 from a top injection line 21, and at a temperature between 1000° C. and 1300° C. at point 22 from a bottom injection line 23.
The specific flow of CO-rich gas required for the top injection line 21 is between 300 and 600 Nm3 per tonne of cast iron and for the bottom injection line 23, it is between 200 and 500 Nm3 per tonne of cast iron.
The difficulty of this configuration is in controlling these flows. Indeed, the CO-rich gas that circulates in the bottom injection line 23 and top injection line 21 is at a temperature above 700° C. for the top injection line and above 1000° C. for the bottom injection line, and it is therefore not possible to use conventional control valves since the latter do not withstand such temperatures, in particular in lines for circulating reducing gas.